1. Field of the Invention
This invention relates to fine line electrical cables. More particularly, the invention relates to flexible, fine line electrical cables which are manufactured by interleaving two electrical cable subassemblies.
2. Description of the Related Art
Electronic components often include flexible electrical cables for directing electricity to various subcomponents. Such flexible electrical cables are typically manufactured by creating electrical circuit traces etched from copper clad laminates and/or copper bond to a flexible substrate. A flexible insulating material such as polyimide is often selected as the substrate. A sheet of conductive material is laminated to the flexible substrate by using an adhesive such as phenolic butyral, acrylic or modified epoxy. Electrical traces are created in the conductive material lithographically. A photoresist is intimately placed upon the conductive material, exposed through a mask, and then the undeveloped photoresist is selectively removed from the areas between the desired traces. A chemical etchant which is relatively active upon the conductive material, but relatively inactive upon the photoresist, is then used to selectively remove the conductive material from the areas between the desired traces. The remaining photoresist is then stripped away, leaving only the substrate, adhesive, and the pattern of conductive traces thereon. The pattern is typically a set of parallel traces. A second layer of insulating material is then placed upon the conductive traces to completely insulate them, using additional adhesive as required.
Another known method of manufacturing flexible electrical cables begins with the deposition of an interface layer, such as chromium, upon a flexible substrate, such as polyimide. A thin layer of conductive material, such as copper is then deposited upon the interface layer. The interface layer and the conductive material may be deposited by sputtering. The interface layer ensures that the conductive material adequately adheres to the flexible substrate. Electrical traces are created lithographically. A photoresist is intimately place upon the thin layer of conductive material, exposed through a mask, and then the underdeveloped photoresist is selectively removed from the areas of the desired traces. A thick layer of the same conductive material is plated into the areas of the desired traces and the remaining photoresist is stripped away. The interface layer and thin layer of conductive material are then removed by chemical etching down to the substrate in the space between the thick conductive layer areas of the desired traces. The remaining electrical traces are thus composed of three layers added to the flexible substrate. Again, a second layer of insulating material is then placed upon the conductive traces to completely insulate them.
The aforementioned methods of manufacture are acceptable for many flexible electrical cable applications. However, the spacing between adjacent traces is limited by the resolution of the lithographic techniques used to remove the conductive material therebetween. Modern electronic components require increased concentrations of electrical traces (i.e. decreased spacing between traces). Thus, a heretofore unrecognized problem is how to achieve electrical traces in flexible cables at spacings less than those made possible by the resolution of the lithography technology.
Occasionally it is necessary to cross traces in flexible circuit cables. The crossed traces must be insulated from each other to prevent electrical shorting. Additional layers of conductive and insulating material are typically used to cross the traces. After the aforementioned patterning of the traces and stripping of the photoresist, additional patterning is used to raise a trace into another plane to allow for the crossover. Similar patterning may also be used to return the raised trace to its original plane. Such additional layers of material and patterning make the manufacture of the flexible electrical cables more complex, time consuming, and expensive. Such additional patterning may also include the use of conductive thru holes which employ plating, riveting, or other techniques to electrically connect two planes. Thru holes cannot be manufactured to have a diameter the same or less than the minimum electrical trace width made possible by the resolution of the lithography technology. The use of thru holes, therefore, increases the permissible width and spacing of electrical traces. Thus, another heretofore unrecognized problem is how to achieve simple trace crossovers in flexible electrical cables.